Summary: We have continued to study the role of host factors in CD4+ T lymphocyte depletion following HIV infection. The cytokine TGF-b1 was found to synergistically induce CD4+ T cell death during macrophage-tropic HIV infection of resting peripheral blood CD4+ T cells in vitro. Most CD4 cells in these cultures have a resting cell phenotype CD25-CD69-DR-Ki67- and were of the memory type, CD45RO+. Cells cultured with TGF-b1 had increased expression of CCR5 and CXCR4 and supported HIV-1 entry and reverse transcription. Virus production was inhibited in the presence of TGF-b1 and was associated with extensive apoptotic T cell death. The synergistic interaction of HIV infection and TGF-b1 in stimulating apoptosis was related to a reduction in the level of Bcl-2 (an anti-apoptotic factor), to an increase in expression of apoptosis-inducing factor (AIF) and caspase-3, and to the cleavage of BID, c-IAP-1, and XIAP. Apoptosis was not prevented by inhibitors of caspases, consistent with a caspase-independent mechanism of cell death. These results further define the role of TGF-b1 in depletion of CD4+ T cells after HIV-1 infection and provide additional evidence that TGF-b1 is a potential regulator of HIV-1 pathogenesis and a possible target for therapeutic intervention. In other studies, we have identified a novel cell surface host factor which regulates early steps in HIV infection. This protein is an amino-acid transporter that is able to generate intracellular signals following crosslinking with specific antibodies. Antibody induced signalling rapidly inhibits the completion of HIV reverse transcription and nuclear translocation of the HIV pre-integration complex. Inhibition of HIV correlates with alterations in specific cytoskeletal proteins that interact with HIV complexes early after virus entry into susceptible cells. These studies may identify novel approaches to controlling HIV infection.